It is foreseeable that in future wireless communications will include voice, interactive data, video and audio on-demand, and digital video and audio broadcasting services. It will be efficient and cost-effective for a single system to provide these services simultaneously. The challenges in developing such a system lie in two aspects of the distinctive transmission characteristics associated with these applications: traffic symmetry and signal interaction environment. For example, voice traffic is symmetric on both the downlink and uplink, whereas interactive-data traffic is asymmetric, with heavier load on the downlink. Voice services are based on unicast, where signals from other base stations are considered as unwanted interference and thus should be minimized, and whereas data broadcast services
In general, existing wireless systems are inadequate in dealing with both aspects of these challenges. Some time-division duplex systems are designed to support asymmetric two-way applications, but are unequipped to carry out broadcasting efficiently. Some systems are designed for broadcast, but are unable to provide two-way applications. There is a need to overcome such underlying difficulties.